As oil and gas wells are drilled in deeper waters and into increasingly deeper formations, the costs to explore, drill, and complete such wells increases at an alarming rate. As such, it becomes increasingly valuable to evaluate the well as early and as frequently as possible in an effort to ensure that production upon completion is maximized.
An important factor in such evaluation throughout the life of the well is reliably communicating measured data between a location down a borehole and the surface. It is desirable to communicate data generated downhole to the surface during operations such as drilling, perforating, fracturing, and drill stem or well testing, such as reservoir evaluation testing, pressure and temperature monitoring; and during production operations. Communication is also desired to transmit intelligence from the surface to downhole tools or instruments to effect, control or modify operations or parameters.
Accurate and reliable downhole communication is particularly important when complex data comprising a set of measurements or instructions is to be communicated, i.e., when more than a single measurement or a simple trigger signal has to be communicated. For the transmission of complex data it is often desirable to communicate encoded analog or digital signals. One example of a situation where complex data is generated downhole is during Drill Stem Testing (DST). A DST string is a temporary completion containing all the tools necessary to perform pressure transient analysis of a reservoir, e.g., gauges, sensors and the like.
One approach which has been widely considered for borehole communication is to use a direct wire connection (“wireline”) between the surface and the downhole location(s). In such cases, wiring is usually provided in the wireline to transmit signals between the surface and the downhole location(s). The wireline can provide power to the downhole instruments, and also control signals to control the operation of the instruments. The wireline can be used to provide a wired communications link for the telemetry of signals between the surface and the downhole location.
Wireless communication systems have also been developed for purposes of communicating data between the downhole location and the surface of the well. These techniques include, for example, communicating commands downhole via (1) pressure or fluid pulses; (2) acoustic communication; and, (3) electromagnetic waves.
Mud pulse telemetry systems function to vary the pressure of mud flowing through the bore of the string and up through the borehole annulus. Such systems may operate a series of valves to selectively communicate the bore and annulus pressures to either add positive pressure pulses to returning drilling mud (positive pulse telemetry) or to add negative pressure pulses to returning drilling mud (negative pulse telemetry).
Acoustic telemetry, on the other hand, uses an oscillator-type device (e.g., piezoelectric transducer) to generate an acoustic signal that is to modulate along the string or through the wellbore fluid. Additionally, acoustic telemetry may be used to provide wireless bi-directional telemetry for transferring information from the surface to the downhole location and vice-versa.
Electromagnetic telemetry systems use a downhole transmitter to send electromagnetic waves (“EM signals”) from the wellbore, up through the string, the casing, the wellbore fluid, and/or the formation adjacent the wellbore, to a receiver located at, or near, the surface of the well. As in the acoustic telemetry systems, electromagnetic telemetry may also be used to provide bi-directional telemetry between the surface and downhole. Such systems include a downhole unit, e.g., a downhole telemetry module that creates an electromagnetic field capable of sending a signal to a remote surface unit or to one or more repeaters positioned between the downhole unit and the surface unit. Advancements, such as the use of repeaters and non-conductive gaps, have been implemented to improve the operability of electromagnetic systems in oilfield applications.
Common methods of implementing an electromagnetic telemetry system involve creating a gap, or non-conductive insert, between adjoining sections of the string and then generating a difference of potential between the two sides of the non-conductive gap. This creates a current that propagates along the string to the surface. An electromagnetic field created by the current propagates a communication signal between a downhole location and the surface, or between downhole locations themselves. Although a generally desirable method of communicating, electromagnetic telemetry also suffers from certain limitations which can limit its application in certain circumstances.
For example, because the communication range of electromagnetic telemetry is highly dependent on current propagation, the reliability and range of communication can be significantly limited when communicating in an area with a low resistivity. For example, the resistivity of the toolstring, the wellbore fluid, the casing, and the formations surrounding the wellbore can affect the electromagnetic signal propagation. In particular, unforeseen formation layers with low resistivity can prevent the electromagnetic signal from reaching the surface, thereby disrupting or even preventing communication between the surface and the downhole location.
Another limitation of electromagnetic telemetry is the effective communication rate. That is, the transfer rate of data can be too slow to communicate data in real time. Generally, systems utilizing electromagnetic systems to transfer data, e.g., data relating to sensed downhole parameters and the like, only transmit a sampling of the data, rather than the complete set of data. These systems often rely on memory modules downhole to record the data which is then downloaded once the tool is recovered to the surface. It would be desirable to obtain the complete set of downhole data, and obtain the complete set of downhole data at a higher transfer rate.
Despite the efforts of the prior art, there exists a need to ensure reliable communication of data between a downhole location and a surface location.